Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Mask inspection apparatus are used for the inspection of reticles for microlithographic projection exposure apparatus.
In projection lenses or inspection lenses designed for the extreme ultraviolet (EUV) wavelength range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, reflective optical elements are used as optical components for the imaging process.
Here it is known, inter alia, to provide a wavefront correction layer on a reflective optical element, the thickness profile of said wavefront correction layer typically being set subsequently in order to correct a wavefront aberration that occurs during the operation of the optical system. Furthermore, the use of a film element operated in transmission as an additional optical wavefront correction element, said film element causing a desired wavefront correction by way of a varying layer thickness profile, is also known.
A problem occurring in practice with the approach mentioned last is that an ablation or addition of carrier material of the film element, undertaken for producing the varying layer thickness profile, may have a mechanical destabilization effect as a consequence.
A further problem occurring in the approaches described above is that, where necessary, a protective layer (e.g. an anti-oxidation protective layer) that is required in practice is still subsequently applied, or applied after producing the respective varying layer thickness profile, as a result of which, in turn, the accuracy of the wavefront modification that is ultimately provided by the wavefront correction element in the optical system is impaired.
With regard to the background art, reference is made by way of example to WO 2013/117343 A1.